Protection and control systems are employed in a variety of industrial installations including, for example, nuclear reactor power generating plants. Such systems must operate accurately with a high degree of reliability.
It is known to periodically test each channel by supplying to the channel input a signal simulating a malfunction or other condition to which the system should produce a response and observing whether such response is produced.
In certain cases, such as in nuclear reactor power generating installation, the reliability of response to an abnormal condition, which includes avoidance of responses to spurious abnormal condition indications, is improved by monitoring a particular critical condition independently in several, e.g. four, monitoring channels and by comparing the control signals at the outputs of the channels to determine whether an abnormal condition has in fact occurred. By way of example, if any two of the four channels provide output signals indicative of an abnormal condition, it is assumed that the abnormal condition does exist and the system is caused to respond, as by producing a reactor shutdown. Thus, an abnormal condition output signal in one channel is treated as spurious.
It is current practice to manually test such systems by a procedure which takes up to two hours per channel and which involves applying to the channel under test an input signal representing the faulty condition to which that channel is responsive. The appearance of an abnormal condition output signal at the output of the channel under test indicates that the channel is operational.
However, in addition to the fact that such testing procedure is extremely time consuming, this procedure has the drawback that if, during the test, any other associated channel produces a spurious abnormal condition signal, the system will respond as if the abnormal condition really existed.